JP5503387B2 - Photoconductive element and imaging device - Google Patents

Description

  The present invention relates to a photoconductive element and an imaging device capable of obtaining a high-quality image with high sensitivity and high resolution and high S / N.

Conventionally, it is known that when a high electric field of about 1 × 10 ⁸ [V / m] or more is applied to a photoconductive layer composed of a selenium-based amorphous semiconductor, an avalanche multiplication phenomenon occurs inside. A highly sensitive imaging tube using this phenomenon has been proposed (see, for example, Non-Patent Document 1).

In order to generate such an avalanche multiplication phenomenon, it is necessary to apply a high electric field of about 1 × 10 ⁸ [V / m] or more to the photoconductive layer. It is necessary to suppress the dark current by preventing the injection of charges (holes) into the photoconductive layer.

As a technique for preventing such charge (hole) injection, for example, a cerium oxide (CeO ₂ ) thin film is blocked between the positive electrode and the selenium-based amorphous semiconductor (photoconductive layer). A method of providing a layer is known (see, for example, Patent Document 1).

JP 09-050771 A

Proceedings of the annual conference of the Institute of Television Engineers of Japan, 17-18 pages, 1989

However, even if a cerium oxide (CeO ₂ ) thin film is provided as the hole injection blocking layer as described above, depending on the film quality of the hole injection blocking layer and the strength of the applied electric field, the selenium-based amorphous material In some cases, injection of charge into the semiconductor (photoconductive layer) cannot be effectively suppressed, and dark current may be generated due to charge injection that has passed through the hole injection blocking layer.

  In general, in a high-sensitivity image pickup tube using the avalanche multiplication phenomenon, a higher multiplication factor (sensitivity) can be obtained as the electric field applied to the photoconductive layer is higher. The dark current due to the avalanche multiplication may also cause image quality deterioration such as S / N reduction.

  For this reason, there is a limit to the intensity of the electric field that can be applied to the photoconductive layer, and there is a problem that the maximum value of the multiplication factor of the high-sensitivity image pickup tube is limited by the limit of the electric field intensity.

  Therefore, the present invention has an object to provide a photoconductive element and an imaging device including a hole injection blocking enhancement layer capable of suppressing dark current and obtaining a high-quality image with high sensitivity and high resolution and high S / N. To do.

The photoconductive element according to one aspect of the present invention includes a translucent substrate, a conductive film formed on the translucent substrate, a hole injection blocking layer formed on the conductive film, and the hole. A photoconductive layer formed on the injection blocking layer, wherein the hole injection blocking layer is made of zinc oxide, and the photoconductive layer is made of an amorphous semiconductor layer mainly composed of selenium. .

The photoconductive layer may be formed of a semiconductor layer that causes an avalanche multiplication phenomenon .

  An imaging device according to an aspect of the present invention includes the photoconductive element described above, an electron beam source that emits a scanning electron beam to the photoconductive element, and the electron conductive element that is electrically connected to the photoconductive element. A signal reading unit for reading an imaging signal obtained by beam scanning.

  According to the present invention, it is possible to provide a photoconductive element and an imaging device including a hole injection blocking enhancement layer that can suppress dark current and obtain a high-sensitivity image with high sensitivity, high resolution, and high S / N. Is obtained.

1 is a diagram illustrating a configuration of an imaging device according to a first embodiment. 3 is a cross-sectional view illustrating in detail a configuration of a photoconductive portion 13 of the imaging device according to Embodiment 1. FIG. It is a figure which compares and shows the band structure of the hole injection blocking layer made of zinc oxide of the photoconductive element of Embodiment 1 and the band structure of the cerium oxide hole injection blocking layer of the conventional photoconductive element. It is a figure which shows the characteristic of the signal current and dark current of the imaging device of Embodiment 1, and the characteristic of the signal current and dark current of the conventional imaging device. 6 is a diagram illustrating a cross-sectional structure of an imaging device according to Embodiment 2. FIG.

  Hereinafter, embodiments to which the photoconductive element and the imaging device of the present invention are applied will be described.

[Embodiment 1]
1A and 1B are diagrams illustrating a configuration of an imaging device according to Embodiment 1, in which FIG. 1A is a side cross-sectional view, and FIG. 1B is a diagram illustrating the imaging device in a cross-sectional view taken along line AA in FIG.

  As shown in FIG. 1A, the imaging device 100 according to the first embodiment includes a photoconductive element 10, an indium ring 20, a glass tube 30, an electron beam source 40, a deflection device 50, a mesh electrode 60, and a signal readout device. 70.

  The photoconductive element 10 includes a translucent substrate 11, a conductive film 12, a photoconductive portion 13, and a metal pin 14. The translucent substrate 11 is a transparent substrate that transmits the reflected light obtained from the subject and guides it to the photoconductive portion 13, and is composed of, for example, a disc-shaped substrate made of translucent glass and having a diameter of 18 mm. . A conductive film 12 having a diameter of 14 mm and a thickness of 30 nm mainly composed of indium oxide is formed on one surface of the translucent substrate 11 by a sputter deposition method.

  A photoconductive portion 13 is formed on the lower surface of the conductive film 12 in the drawing. The metal pin 14 is fitted into a through-hole penetrating the translucent substrate 11, one end is electrically connected to the conductive film 12, and the other end is connected to the signal readout device 70.

  The photoconductive portion 13 becomes a target of an electron beam emitted from the electron beam source 40, and causes an avalanche multiplication phenomenon inside by applying a high electric field. The photoconductive portion 13 has a circular shape in plan view, and has a scanning surface 13A scanned by the electron beam 41, and an incident surface 13B through which light enters from the upper direction in the drawing through the translucent substrate 11 and the conductive film 12. . The incident surface 13 </ b> B is a boundary surface between the photoconductive portion 13 and the conductive film 12.

  Further, as shown in FIG. 1B, the photoconductive portion 13 has a scanning region 13C at the center of the scanning surface 13A facing the electron beam source 40. The scanning region 13C is a region where scanning for taking out an imaging signal is performed by the electron beam 41 deflected and focused by the deflection coil and the focusing coil. The metal pin 14 is disposed at a position outside the scanning region 13C in plan view. The detailed configuration of the photoconductive portion 13 will be described later with reference to FIG.

  The indium ring 20 seals between the translucent substrate 11 and the glass tube 30, and fixes the photoconductive element 10 to the glass tube 30.

  The glass tube 30 is a glass tubular member whose one end is sealed by the photoconductive element 10 and the indium ring 20, and the electron beam source 40 is formed inside the glass tube 30 from the back to the opening. The deflection device 50 and the mesh electrode 60 are disposed.

  The glass tube 30 is sealed (sealed) with the translucent substrate 11 by the indium ring 20 in order to realize scanning with an electron beam, so that the internal space is maintained in a vacuum.

  The electron beam source 40 is an electron emission source disposed in the inner part of the glass tube 30 and is an apparatus that excites an electron cloud by heating a cathode material with a heater. Electrons excited as an electron cloud are emitted as an electron beam 41.

  The deflection device 50 includes a first grid electrode 51, a second grid electrode 52, and a third grid electrode 53, and deflects the electron beam 41. The first grid electrode 51, the second grid electrode 52, and the third grid electrode 53 are electrodes for extracting and accelerating electrons excited by the electron beam source 40 as electron beams 41 in the direction of the mesh electrode 60.

  The mesh electrode 60 is provided in front of the photoconductive portion 13 when viewed from the electron beam source 40, and is a deceleration for reducing the speed of electrons that collide with the photoconductive portion 13 that is the target of the scanning electron beam 41. It is an electrode that generates an electric field.

  The signal readout device 70 is a device for reading out the electric charge generated in the photoconductive portion 13 as an imaging signal, and includes a power source 71 for applying a voltage to the photoconductive portion 13 and a readout portion 72 for reading out the imaging signal. Have

  A positive terminal of the power supply 71 is connected to the photoconductive portion 13 via the metal pin 14 and also connected to the readout portion 72. Further, the negative terminal of the power source 71 is connected to the electron beam source 40, and a closed circuit is formed via the scanning electron beam 41. Here, the voltage applied from the power source 71 to the photoconductive portion 13 is referred to as a target voltage. For convenience of explanation, the line connecting the negative terminal of the power source 71 and the electron beam source 40 is not shown.

  A deflection coil and a focusing coil (not shown) are disposed outside the glass tube 30 to deflect and focus electrons (electron beam 41) accelerated in the third grid electrode 53.

  In the imaging device 100 according to the first embodiment, the voltage applied to each electrode is, for example, the heater of the electron beam source 40: about 6V, the first grid electrode 51: about 20V, and the second grid electrode 52: about 300V, the third grid electrode 53: about 600V, and the mesh electrode 60: about 800V.

  A target voltage of 100 to 5000 V is applied to the light incident surface 13B of the photoconductive portion 13 from the power source 71 via the conductive film 12 according to the thickness of the photoconductive portion 13.

  The surface potential of the scanning region 13C becomes the same potential as that of the electron beam source 40 immediately after the end of scanning, and the same voltage as the target voltage is applied between the incident surface 13B of the photoconductive portion 13 and the scanning surface 13A. Thereafter, when light is incident, electron-hole pairs are generated in the photoconductive portion 13 by the absorbed light, and the holes travel to the scanning surface 13A along the electric field in the photoconductive portion 13, and the scanning surface 13A. The surface potential changes in the positive direction. The surface potential of the scanning surface 13A that has risen in the positive direction returns to the same potential as that of the electron beam source 40 again by the next electron beam scanning. At this time, a change in current flowing in the closed circuit is taken out from the reading unit 72 as an imaging signal. .

  Electrons generated by the electron beam source 40 are extracted by the first grid electrode 51, emitted as the electron beam 41 by the second grid electrode 52, deflected and focused by an external magnetic field, and accelerated by the third grid electrode 53. The The accelerated electrons are decelerated when passing through the mesh electrode 60 and reach the surface of the photoconductive portion 13 at a substantially zero speed. Thereby, it can suppress that an electron collides with the photoconductive part 13 at high speed, and discharge | release of the secondary electron by collision is suppressed.

  Next, the configuration of the photoconductive portion 13 will be described with reference to FIG.

  FIG. 2 is a cross-sectional view illustrating in detail the configuration of the photoconductive portion 13 of the imaging device according to the first embodiment.

  The photoconductive portion 13 is a stacked body of a hole injection blocking layer 131, a photoconductive layer 132, and an electron beam landing layer 133, and is stacked on the surface of the conductive film 12 of the translucent substrate 11.

  The hole injection blocking layer 131 is a layer for blocking the injection of charges (holes) from the conductive film 12 to the photoconductive layer 132, and is made of zinc oxide (ZnO) formed to suppress dark current. It is a thin film layer. The hole injection blocking layer 131 is formed on the surface of the conductive film 12 by a vacuum deposition method.

  The photoconductive layer 132 is an amorphous semiconductor layer containing selenium (Se) as a main material, and is a semiconductor layer that causes an avalanche multiplication phenomenon when a high voltage is applied. This photoconductive layer 132 is formed on the surface of the hole injection blocking layer 131 by vacuum deposition.

  The electron beam landing layer 133 is a layer that is emitted from the electron beam source 40, accelerated by the first grid electrode 51, the second grid electrode 52, and the third grid electrode 53, and reaches the electrons decelerated by the mesh electrode 60. is there.

  Next, a method for producing the photoconductive portion 13 will be described.

  Such a photoconductive portion 13 is manufactured as follows. First, a hole injection blocking layer 131 made of zinc oxide (ZnO) having a diameter of 14 mm and a film thickness of 10 nm is formed on the conductive film 12 by vacuum deposition. At this time, the temperature of the translucent substrate 11 may be maintained at, for example, about 200 ° C. (or higher temperature) by using a heating mechanism provided in the vacuum evaporation apparatus.

Next, a photoconductive layer 132 made of an amorphous semiconductor mainly composed of selenium (Se) having a diameter of 14 mm and a film thickness of 2 to 50 μm is formed on the hole injection blocking layer 131 by a vacuum deposition method. Further, antimony trisulfide (Sb ₂ S ₃ ) is deposited in an argon (Ar) gas atmosphere at a pressure of 0.1 to 0.4 Torr to form an electron beam landing layer 133 having a diameter of 14 mm and a film thickness of 0.1 μm. The photoconductive portion 13 is obtained as described above.

  By sealing the glass substrate 30 containing the translucent substrate 11 with the photoconductive portion 13 formed in this way, the electron beam source 40, the mesh electrode 60 and the like with the indium ring 20, and vacuum-sealing the inside. The imaging device of Embodiment 1 is obtained.

Next, a band structure of the hole injection blocking layer 131 made of zinc oxide (ZnO) of the photoconductive element 10 of Embodiment 1 and the hole injection blocking layer made of cerium oxide (CeO ₂ ) of the conventional photoconductive element. Will be described.

FIG. 3A is a diagram schematically showing the band structure of the hole injection blocking layer 131 made of zinc oxide (ZnO) of the photoconductive element 10 of Embodiment 1, and FIG. It is a figure which shows typically the band structure of the hole-injection blocking layer made from cerium oxide (CeO ₂ ) of the photoconductive element. Note that the band structure shown in FIG. 3 evaluates the ionization potential (energy difference (WF) between the upper end of the valence band and the vacuum level) from the ultraviolet photoelectron spectroscopy (UPS) spectrum, and determines the vacuum level (V.V.). L.), the valence band (Ev), conduction band (Ec), and Fermi level (Ef) are obtained. The band gap (Eg) is a value calculated from an ultraviolet photoelectron spectroscopy (UPS) spectrum and an EELS (Electron Energy Loss Spectroscopy) spectrum at the K-edge of oxygen.

As shown in FIG. 3A, in the hole injection blocking layer 131 made of zinc oxide (ZnO), the energy level difference between the Fermi level (Ef) and the valence band (Ev) is 3. On the other hand, in the hole injection blocking layer made of cerium oxide (CeO ₂ ), as shown in FIG. 3B, it is between the Fermi level (Ef) and the valence band (Ev). The difference in energy level was 3.02 eV. The work function (WF) and ionization potential (IP) of zinc oxide (ZnO) and cerium oxide (CeO ₂ ) were as shown in FIGS. 3 (A) and 3 (B), respectively. .

The difference in energy level between the Fermi level (Ef) and the valence band (Ev) represents the size of the barrier against holes (hole barrier). Therefore, by using a zinc oxide (ZnO) layer having a hole barrier larger than that of cerium oxide (CeO ₂ ) as the hole injection blocking layer 131 as described above, the selenium-based non-conductive layer 12 is removed from the conductive film 12 which is a positive electrode. It is expected that charge injection into the photoconductive portion 13 made of a crystalline semiconductor can be effectively suppressed.

  Next, characteristics of the signal current and dark current of the imaging device 100 according to the first embodiment will be described.

  FIG. 4 is a diagram illustrating the characteristics of the signal current and the dark current with respect to the applied voltage in the imaging device 100 according to the first embodiment, and the characteristics of the signal current and the dark current of the conventional imaging device.

The conventional imaging device includes a hole injection blocking layer made of cerium oxide (CeO ₂ ) instead of the hole injection blocking layer 131 made of zinc oxide (ZnO) in the imaging device 100 of the first embodiment. The applied voltage is a voltage applied to the photoconductive portion 13 through the metal pin 14 and the conductive film 12. The signal current is indicated by an absolute value (au (arbitrary unit)), and the dark current is indicated by (nA).

  As shown in FIG. 4, the signal current increases to about 30000 (au) in the imaging device 100 according to the first embodiment, while the maximum is about 5000 (au) in the conventional imaging device. I understand that.

  The dark current is about 1 (nA) at the minimum in the conventional image sensor, and the dark current value in the range up to about 40 (nA) with the increase of the applied voltage is shown. It can be seen that the imaging device 100 of aspect 1 shows a dark current value in a range of about 0.35 (nA) at the minimum and up to about 12 (nA) as the applied voltage increases.

Thus, according to the imaging device 100 of Embodiment 1 using the zinc oxide (ZnO) layer as the hole injection blocking layer 131, the signal current is dramatically increased as compared with the conventional imaging device (FIG. 4). In this example, the dark current is drastically reduced (about 1/3 or less). For this reason, the S / N dramatically increases. As shown in FIG. 3, the dark current was reduced in this way because a zinc oxide (ZnO) layer having a hole barrier larger than cerium oxide (CeO ₂ ) was used as the hole injection blocking layer 131. Is considered to be a cause.

  As described above, according to the first embodiment, by using the zinc oxide (ZnO) layer produced while heating the translucent substrate 11 as the hole injection blocking layer 131, the dark current is greatly reduced, and high sensitivity / A photoconductive imaging device capable of obtaining a high-definition image with high resolution and high S / N can be provided.

Among imaging devices, particularly in an imaging device such as a high-sensitivity imaging tube using the avalanche multiplication phenomenon, it is necessary to apply a high electric field of, for example, about 1 × 10 ⁸ [V / m] or more to the photoconductive layer. is there. For this reason, in particular, in an imaging device such as a high-sensitivity imaging tube using the avalanche multiplication phenomenon, the use of a zinc oxide (ZnO) layer as a hole injection blocking layer provides a remarkable dark current suppression effect. High-quality images with high sensitivity and high resolution and high S / N can be obtained.

In the above, the imaging device using the photoconductive layer made of an amorphous semiconductor mainly composed of selenium (Se) has been described. However, the present invention does not place any restrictions on the material of the photoconductive layer, The present invention can also be applied to an imaging device using a photoconductive layer composed of Si, PbO, CdS, CdSe, CdTe, Sb ₂ S ₃ or the like.

  The translucent substrate 11 is not limited to a glass substrate, and may be a translucent resin substrate or an optical fiber plate. Further, if a thin plate such as beryllium (Be), crystalline silicon (Si), or boron nitride (BN) having a high X-ray transmittance is used, the dark current of the X-ray imaging device can be reduced. .

  In the above description, the image pickup tube that is generally widely used has been described. However, any device that accelerates an electron beam in a vacuum and lands on a photoconductive layer to read out accumulated charges can be used. It may be an imaging device of the form.

[Embodiment 2]
FIG. 5 is a diagram illustrating a cross-sectional structure of the imaging device according to the second embodiment. The imaging device 200 of the second embodiment is suitable for an X-ray imaging device, and is mainly different from the imaging device of the first embodiment in that an electron emission source array 240 is provided as an electron beam source. The electron emission source array 240 has a plurality of minute cathodes arranged in a matrix. An arbitrary cathode can be selected to emit an electron beam. Therefore, unlike the imaging device 100 according to the first embodiment, the first grid electrode 51, the second grid electrode 52, the third grid electrode 53, the deflection coil, and the focusing coil are not provided.

  In the second embodiment, the glass tube 230 has a cup shape. The electron emission source array 240 is disposed on the bottom surface of the glass tube 230.

  Other configurations are basically the same as those of the imaging device according to the first embodiment. Therefore, the same or equivalent components are denoted by the same reference numerals, and description thereof is omitted.

  In the imaging device of the second embodiment, a thin plate-like conductive substrate made of beryllium (Be) having a thickness of 0.5 mm and a diameter of 18 mm is used as the translucent substrate 11. Since the thin plate-like translucent substrate 11 made of beryllium (Be) has conductivity, it is not necessary to form the conductive film 12 on one surface as in the first embodiment.

  The photoconductive portion 13 is formed on one surface of the translucent substrate 11 as in the first embodiment. The translucent substrate 11 is sealed with the glass tube 230 through the indium ring 20.

  Moreover, as described above, the glass tube 230 of the second embodiment is cup-shaped. An electron emission source array 240 is disposed on the bottom surface inside the cup-shaped glass tube 230, and a mesh electrode 60 is disposed in the vicinity of the opening 230A.

  The electron emission source array 240 is a flat electron beam source in which a plurality of minute cathodes are arranged in a matrix. By controlling the voltage value applied to the scan line and the selection line, the electron beam 241 can be emitted from the desired cathode to the photoconductive portion 13.

  The translucent substrate 11 having the photoconductive portion 13 formed on the surface as described above and the glass tube 230 containing the electron emission source array 240, the mesh electrode 60, etc. are sealed using the indium ring 20, and the inside is sealed. By performing vacuum sealing, the imaging device of Embodiment 2 can be manufactured.

  The relationship between the voltage applied to the photoconductive portion 13 and the imaging signal current and dark current of the imaging device depends on the structure and film thickness of the photoconductive portion 13. Therefore, also in the imaging device of the second embodiment using the same photoconductive portion 13 including the hole injection blocking layer 131 and the photoconductive layer 132 made of zinc oxide (ZnO) as in the first embodiment, the embodiment As in the case of the first imaging device, the dark current can be reduced as compared with the imaging device according to the prior art.

  In the above description, the glass tube 230 has a cup shape. However, instead of the cup-shaped glass tube 230, a metal vacuum chamber having the same shape may be used.

  The imaging devices of Embodiments 1 and 2 described above are most suitable for television cameras that are required to have high image quality, particularly high-definition cameras. Effects such as signal processing with a high S / N are obtained.

  Although the photoconductive element and the imaging device according to the exemplary embodiments of the present invention have been described above, the present invention is not limited to the specifically disclosed embodiments and deviates from the scope of the claims. Various modifications and changes can be made without this.

DESCRIPTION OF SYMBOLS 100 Imaging device 10 Photoconductive element 11 Translucent substrate 12 Conductive film 13 Photoconductive part 13A Scanning surface 13B Incident surface 13C Scanning region 131 Hole injection blocking layer 132 Photoconductive layer 133 Electron beam landing layer 14 Metal pin 20 Indium ring DESCRIPTION OF SYMBOLS 30 Glass tube 40 Electron beam source 41 Electron beam 50 Deflector 51 1st grid electrode 52 2nd grid electrode 53 3rd grid electrode 60 Mesh electrode 70 Signal read-out device 71 Power supply 72 Read-out part 200 Imaging device 230 Glass tube 240 Electron emission source Array 241 Electron beam

Claims (3)

A translucent substrate;
A conductive film formed on the translucent substrate;
A hole injection blocking layer formed on the conductive film;
A photoconductive layer formed on the hole injection blocking layer,
The hole injection blocking layer is composed of zinc oxide ,
The photoconductive layer is a photoconductive element composed of an amorphous semiconductor layer mainly composed of selenium . The photoconductive element according to claim 1, wherein the photoconductive layer is a semiconductor layer that causes an avalanche multiplication phenomenon . The photoconductive element according to claim 1 or 2,
An electron beam source for emitting a scanning electron beam to the photoconductive element;
An image pickup device comprising: a signal readout unit that is electrically connected to the photoconductive element and reads out an image pickup signal obtained by scanning the electron beam.

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